President Barack Obama will meet with families of the victims of America's largest mass shooting on Sunday.

The White House said Mr Obama would travel to Aurora, Colorado where James Holmes, 24, is alleged to have opened fire during a packed midnight screening of Batman film "The Dark Knight Rises."

The attack has revived the debate over gun control in the United States, and drew condemnation by Mr Obama and his Republican White House rival Mitt Romney.

Both Mr Obama and Mr Romney have cut short their campaign schedules and pulled advertising in Colorado out of respect for the victims and their families.

The actor who plays the caped crusader in the film also voiced sorrow at the shooting.

"Words cannot express the horror that I feel," British actor Christian Bale said in a statement. "I cannot begin to truly understand the pain and grief of the victims and their loved ones, but my heart goes out to them."

Authorities revealed the names of the 12 people who died in Friday's shootings, including a six-year-old girl whose mother was also injured, and two US servicemen. A further 58 people were wounded.

Bomb experts spent Saturday inching their way into Holmes's apartment. A small boom from a controlled detonation was heard outside the apartment block and pieces of debris were blown out of one of the windows.

Aurora police chief Dan Oates did nothing to hide his anger at what was found inside the apartment.

He said: "Make no mistake, this apartment was designed to kill whoever entered it. And who was most likely to enter that location after he planned and executed this horrific crime? It was going to be a police officer.

"And if you think we're angry, we sure as hell are angry, about what has happened to our city."

Officers began covering the windows of the apartment with black plastic to prevent onlookers from seeing in. Before they did, a poster was visible on a closet that advertised a DVD called "Soldiers of Misfortune."

The 12 fatalities included six-year-old Veronica Moser-Sullivan. "She was beautiful and innocent," the girl's great-aunt Annie Dalton said. Her mother Ashley, 25, was shot in the neck and abdomen and is in critical condition.

Police said Holmes planned the attack over four months. He bought more than 6,000 rounds of ammunition online, as well as four guns in the two months before the rampage.